Optical sensor systems that can identify and detect the concentration of various materials in a sample, have a great number of applications in the fields of biotechnology and chemical analysis. High sensitivity pollution sensors, for example, are needed to detect the presence of contaminants in water supplies and in the air we breath. A wide range of benefits can be realized in the biotechnology field if a sensor could detect the concentration of viruses and bacteria in a sample of blood or identify various proteins.
Typical biological sensors (biosensors) require fluorescent chemical compounds which are used to “tag” various protein or DNA molecules. When such molecules are deposited onto a surface through attachment or binding to “receptor” chemicals, the presence and concentration of the tagged chemicals can be determined by the brightness of the light emitted by the fluorescent compounds. Recently, this technique gained prominence due to the rapid sequencing of the human genome.
Fluorescence-based biosensors are limited to the extent that the fluorescent compounds used are not applicable to all materials, such as the huge number of proteins generated in the human body. In many applications, a sensitive high speed detection method is needed which does not require the use of fluorescent chemical tags. Such a method can be realized using an optical resonance phenomenon such as the highly complex surface plasmon resonance (SPR), or by ellipsometry, reflectometry, or grating couplers. None of these methods has been developed to the point where high throughput high sensitivity sensors are compatible with long-established diagnostic or research methods such as the use of microtiter plates. Using a surface structure resonance phenomena, the optical filters described herein can be used to produce biosensors capable of detecting minute concentrations of chemicals through a shift in the wavelength of light resonated from the sensor's surface. This type of response cannot be obtained from other filtering methods such as thin-film interference filters and fiber Bragg grating filters.
There are two types of surface structures which can produce the optical resonance signal suitable for chemical detection as disclosed herein. The first type is referred to as an “Aztec” structure in the literature and was disclosed and fully described by Cowan in U.S. Pat. Nos. 4,839,250, 4,874,213, and 4,888,260. Aztec surface structures resemble stepped pyramids where each step height corresponds to one half the wavelength of light which will add coherently upon reflection (one full wavelength in transmission). A typical Aztec surface profile is shown in FIG. 1A. An Aztec structure will act to filter a narrow range of wavelengths out of a broad range of input signals. In general, the width of the filtered range of wavelengths decreases with an increasing number of steps in the Aztec micro-structures. Cowan also noted that the range of filtered wavelengths could be shifted when the density of the material surrounding the structures was varied. A second technique for producing a narrow optical resonance is to exploit a surface structure waveguide effect. Here a multiple-step Aztec structure or a simple single-step array of structures such as holes or posts, can be embedded in a region of high refractive index to create a waveguide resonator. A cross section of such a device is shown in FIG. 1B. Due to their wavelength selective nature, such three or two dimensional structures have received great attention in the recent literature, especially in the context of optical telecommunications and optical computing. They are know in the art as “photonic bandgap” crystals and are being developed for confining and directing light into planar channels which mimic electrical circuits. Using two and three-dimensional guided-mode waveguide resonators as filters is less well-known in the art but has been described in the literature. (See Magnusson U.S. Pat. Nos. 5,216,680, 5,598,300, and 6,154,480. Also, S. Peng and G. M. Morris, “Resonant Scattering from two-dimensional gratings”, J. Opt. Soc. Am. A, Vol. 13, No. 5, p. 993, May 1996; R. Magnusson and S. S. Wang, “New Principle for optical filters,” Appl. Phys. Lett., 61, No. 9, p. 1022, August 1992.)
Guided mode surface structure resonators produce exceptionally narrow optical resonances. To generate the resonance effect, all of the dimensions in the surface structures, their height, width, and spacing, must be smaller than the wavelengths of light used in the broadband illuminating light. Because the structures are composed of a material with a higher density than the surrounding medium, a waveguide is created in a direction orthogonal to the propagation direction. A narrow range of wavelengths in the illuminating light will be confined and radially propagate a short distance in the plane of the structures, where it will undergo reflection. Waves traveling radially outward in the plane will interfere with waves reflected from the structures allowing the confined beam to leak out of the plane, propagating in a direction opposite the incident direction. Separating these counter-propagating waves is done by several conventional techniques such as beam splitting cubes, optical circulators, waveguide couplers, or grin lens fiber optic collimators.